Rihanna's tour buses were stopped at the border between Michigan and Canada ... and law enforcement sources tell TMZ ... authorities found weed.
A total of 10 buses were stopped at the Ambassador Bridge, which separates Windsor, Canada and Detroit. Inspectors smelled marijuana on one of the buses and initiated a secondary, bus-to-bus search.
Inspectors then brought drug-sniffing dogs on board, and one of them made a beeline for a passenger and authorities found he was in possession of pot. We're told he was cited with a civil penalty.
Rihanna was not on any of the buses.
The real headline -- they only found a "small amount" of marijuana on one person in one bus. Who woulda thunk?
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